You're about to meet a man who has four wives. And they'll all say they're one big happy family. Polygamists believe their practices will help them get to heaven. And now one family is crusading in court to make their more the merrier lifestyle a legal one. And so far they're winning. Reporter: One husband, four wives, 17 children. I believe in living this lifestyle. We know them as reality TV stars on the hit show "Sister wives." I wouldn't want anything else. Reporter: But now Hollywood's most famous polygamist have taken on a new unlikely role as legal crusaders, embroiled in a courtroom fight over the fate of polygamy. Are you prepared to take this all the way to the supreme court in you have to? Absolutely. Reporter: It is an epic battle. Cody brown and all those wives, Mary, Janelle, robin against the state of Utah after the family flaunted their plural lifestyle on TV, authorities went after them and the browns fled Utah for Nevada. It's brain damage to move this group. Reporter: Then they sued, claiming a first amendment right to live how they want to live. It was really an issue of freedom of expression, freedom of love anybody. You should be able to organize their family according to how they choose. Reporter: And they want the first American polygamy ruling in more than 130 years. A federal judge siding with them in December, declaring a key part of Utah's polyga llygamy law unconstitutional. He can live with his live, just as long as there's only one marriage license. The others are his spiritual wooifs. It's nice to not have felons to our names. Thoufs people in plural marriages in Utah now are free. Reporter: The browns say in their home, everyone is equal. But what about all the other polygamists living in the shadows? Is your family still growing? Yes, it will continue to grow. I may have another lady that comes into the family. I'm pretty sure I'll have more children that will come into the family. And so as the husband and the provider, a lot of what I do is prepare for that. Reporter: I met the collies, all 22 of them, dad, three woo wives, 18 kids last year. They, too, say the women have a say. But do they really? Are you allowed to say as a wife no more, there's no more room? Not really. Because the way I came in was I believed I belonged to Michael and I can't say there's nobody else that belongs to you. That's between Michael and god. Reporter: I've also been inside the bizarre world of convicted pedophile Warren Jeffs and the flds. We're getting close now. Last last fall, I went with the leader's own nephew, young men who have turned against everything their uncle stands for. It didn't take long to get the message we were not welcome. Did they do that because of us? Yeah. Reporter: They have their own reality show, too, breaking the faith where they went back to their own neighborhood to try, they say, to free young women who still live there. I used to be on the god jasquad and I know how they act. Given to Warren Jeffs private security force tailed us. What is he trying to do right now? Intimidate us? Intimidate us. Reporter: Why doesn't a girl who's 20, 21 years old get up and go. They don't know where to go. Only thing they know is their bedroom, the kitsch and the laundry room. And that's it. Reporter: So no fences at all. No. Reporter: The browns live far away from that world and they believe the court ruling in their favor helps the women who still live in it. Let's say a girl is raped by her uncle, okay? And she's in the flds community, she's more concerned about taking this to the law and having her family ripped apart than she is about actually being raped again. This is the truth. This is what's happening. Reporter: Could this ruling that helps your family and families like yours also be helping believers and followers of Warren Jeffs? What we're hoping is this ruling will help create the checks and balances that need to exist in societies that are closed now. They've got to be kidding. Reporter: Lawyer Marcy Hamilton studies polygamy and says the ruling actually makes it tougher for authorities to investigate potential abuse. Reporter: When you have a polygamist culture and you have one man with five women, you have to do two things -- you have to marry younger and younger women in order for one man to have multiple wives. And you have to discard some of the boys. Reporter: The united nations calls polygamy a violation of human rights. Another study concluded polygamy leads to poverty and higher rates of child neglect and abuse. It is a rl attack on the equality of women. Even Cody brown's arrangement is offensive to equality principles. Reporter: But for the browns and other families like theirs, this life, one man, many wives, is a means to get to heaven. And while Utah's attorney general plans to appeal the ruling, the browns say they'll fight for it before a higher court if they have to. I just want to be free man. Reporter: And a higher power. I'm Cecelia Vega in Las Vegas.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Florida school shooting: Details emerge in timeline, what we know about suspect

Now Playing: {{itm.title}}

{"id":22629266,"title":"'Sister Wives' Take on Pro-Polygamy Crusade","duration":"3:00","description":"Kody Brown and his four wives went to court to fight for the right to live openly as a family.","url":"/Nightline/video/sister-wives-pro-polygamy-crusade-22629266","section":"Nightline","mediaType":"default"}